Electroluminescence display apparatus

ABSTRACT

Side faces of anodes have a tapered incline that becomes broader toward a lower layer. Thus, an emissive element layer is smoothly formed on the anodes making it possible to prevent field contraction of the electric field. An EL display apparatus having long life and high yield is provided by preventing the emissive element layer from rupturing between an anode and a cathode and by preventing concentration of the electric field at an upper edge of the anode facing the cathode and localized deterioration in the emissive element layer.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a luminescence display apparatuscomprising electroluminescence elements and thin-film transistors.

[0003] 2. Description of the Related Art

[0004] In recent years, electroluminescence (referred to hereinafter asEL) display apparatuses employing EL elements as emissive elements haveattracted attention as being the display apparatuses to replace CRTs andLCDs, and the research and development also have advanced on EL displayapparatuses comprising thin-film transistors (referred to hereinafter asTFT) as switching elements to drive the EL elements.

[0005]FIG. 1 shows an equivalent circuit of an EL display apparatuscomprising a conventional EL element and TFT.

[0006]FIG. 1 is an equivalent circuit of an EL display apparatuscomprising a first TFT 130, a second TFT 140, and an organic EL element160, and shows the circuitry near a gate signal line Gn of row n and adrain signal line Dm of column m.

[0007] The gate signal line Gn supplying a gate signal and the drainsignal line Dm supplying a drain signal are perpendicular to each other,and near the intersection of both signal lines are provided the organicEL element 160 and the TFTs 130, 140 driving the organic EL element 160.

[0008] The first TFT 130, which is a switching TFT, comprises gateelectrodes 131 connected to the gate signal line Gn and supplied withgate signals, a drain electrode 132 connected to a data signal line(drain signal line) Dm and supplied with data signals, and a sourceelectrode 133 connected to a gate electrode 141 of the second TFT 140.

[0009] The second TFT 140, which is an organic EL element driver TFT,comprises the gate electrode 141 connected to the source electrode 133of the first TFT 130, a source electrode 142 connected to an anode 161of the organic EL element 160, and a drain electrode 143 connected to adriving power supply 150 that is supplied to the organic EL element 160.

[0010] Furthermore, the organic EL element 160 comprises the anode 161connected to the source electrode 142, a cathode 162 connected to acommon electrode 164, and an emissive element layer 163 sandwichedbetween the anode 161 and the cathode 162.

[0011] Furthermore, a storage capacitor 170 is provided with oneelectrode 171 connected between the source electrode 133 of the firstTFT 130 and the gate electrode 141 of the second TFT 140 and anotherelectrode 172 connected to a common electrode 173.

[0012] The driving method of the circuit shown in the equivalent circuitof FIG. 1 will now be described. When the gate signal from the gatesignal line Gn is applied to the gate electrode 131, the first TFT 130turns on. As a result, the data signal from the data signal line Dm issupplied to the gate electrode 141 and the voltage of the gate electrode141 becomes identical to the voltage of the data signal line Dm. Acurrent proportional to the voltage value supplied to the gate electrode141 is then supplied from the driving power supply 150 to the organic ELelement 160. As a result, the organic EL element 160 emits light at anintensity in accordance to the magnitude of the data signal.

[0013] A conventional EL display apparatus will be described next withreference to FIGS. 2, 3A, and 3B. FIG. 2 is a top view showing one pixelof the conventional EL display apparatus. In FIG. 2, a gate signal line51 corresponds to the gate signal line Gn, a data signal line 52corresponds to the data signal line Dm, a driving power supply 53corresponds to the driving power supply 150, an electrode 54 correspondsto the electrode 172 of the storage capacitor 170, and an anode 61corresponds to the anode 161 of the organic EL element 160. The gatesignal lines 51 are arranged in rows and the data signal lines 52 andthe driving supplies 53 are arranged in columns. The storage capacitorand the emissive element layer are arranged within the area thuspartitioned. The storage capacitor is formed from a semiconductor film13 and the electrode 54. The semiconductor film 13 is connected to thedata signal line 52 via a contact C1, and a gate electrode 11 isarranged between a drain 13 d and a source 13 s.

[0014] A semiconductor film 43 is connected to the driving power supply53 via a contact C2, and a gate electrode 41, which is connected to thesemiconductor film 13, is arranged between a drain 43 d and a source 43s. The semiconductor film 43 is connected to the anode 61 of the organicEL element via a contact C3.

[0015]FIG. 3A is a cross-sectional view along line A-A of FIG. 2. On atransparent substrate 10 is formed the semiconductor film 13, on whichis covered with and formed a gate insulating film 12. On the gateinsulating film 12 are formed gate electrodes 11, which branch from thegate signal line 51, and the storage capacitor electrode 54, on which iscovered with and formed an interlayer insulating film 15. On theinterlayer insulating film 15 is arranged the data signal line 52, whichconnects to the semiconductor film 13 via the contact C1. On these iscovered with and formed a planarization insulating film 17.

[0016]FIG. 3B is a cross-sectional view along line B-B of FIG. 1. On thesubstrate 10 are laminated in sequence the semiconductor film 43, thegate insulating film 12, the gate electrode 41, and the interlayerinsulating film 15, and on the interlayer insulating film 15 are formedthe data signal line 52 and the driving power supply 53, on which iscovered with and formed the planarization insulating film 17. On theplanarization insulating film 17 is arranged an anode 61, which isconnected to the semiconductor film 43 via the contact C3. On the anode61 is arranged an emissive element layer 66, which has a laminatedstructure of a first hole transport layer 62, a second hole transportlayer 63, an emissive layer 64, and an electron transport layer 65. Acathode 67 is arranged so as to cover them.

[0017] The anode 61 of the pattern shown in FIG. 2 is generally formedusing a method in which an ITO film is first formed on the entiresurface, and after forming a positive photoresist in a predeterminedshape, wet etching is performed using chemicals.

[0018] However, when forming the organic EL element in this manner, theemissive element layer 66 that is formed on the anode 61 is extremelythin at approximately 200 nm so that coverage at the step portion withthe planarization insulating film 17 at the edge of the anode 61deteriorates. Thus, at the points indicated by the arrows in FIG. 4,since the vertex of the anode 61 and the vertex of the cathode 67 faceeach other in closer proximity than at any other location, fieldconcentration occurs here causing a problem where the emissive layer 64positioned between layers deteriorates rapidly. As the coveragedeteriorates further, the emissive element layer 66 ruptures as shown inthe figure, and the cathode 67 provided on the upper layer shorts withthe anode 61 to possibly cause this pixel to be defective and notdisplay.

SUMMARY OF THE INVENTION

[0019] It is therefore an object of the present invention to provide anEL display apparatus having long life and high yield by preventingshorts or localized deterioration of the emissive layer 64 due to thethickness of the anode.

[0020] The present invention solves the aforementioned problem and is anelectroluminescence display apparatus comprising an emissive element (anelectroluminescence element) laminated in sequence on the substrate withthe first electrode, the emissive element layer (such as hole transportlayer, emissive layer, and electron transport layer), and the secondelectrode, with the side faces of the first electrode inclined andbecoming broader toward the substrate side.

[0021] The angle formed by the incline of the first electrode and theplane of the lower layer (and/or the substrate) is 10° to 45°, orfurther an angle of 25° to 35°. Furthermore, the side of the firstelectrode has a tapered shape becoming broader from the emissive elementlayer toward the substrate.

[0022] Furthermore, the thickness of the first electrode is less than ½,or further less than ⅓ the total film thickness of the hole transportlayer, the emissive layer, and the electron transport layer.

[0023] As described above, the edge of the first electrode in thepresent invention is inclined so that the electroluminescence elementthat is formed thereon is formed smoothly, shorting of the firstelectrode and the second electrode is prevented, and anelectroluminescence display apparatus having a high yield is obtained.

[0024] Furthermore, since concentration of the electric field at theedge of the first electrode is prevented, the electroluminescenceelement is prevented from locally deteriorating, and a luminescencedisplay apparatus having a long life is obtained.

[0025] Since the angle formed by the incline of the first electrode withthe plane of the lower layer (and/or the substrate) is 10° to 45°, orfurther an angle of 25° to 35°, this ensures the emissive element layercan be formed without loss of reproducibility of the shape of the firstelectrode.

[0026] Furthermore, since the thickness of the first electrode is lessthan ½, or further less than ⅓, the film thickness of the emissiveelement layer, this ensures the emissive element layer can be formed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is an equivalent circuit diagram of an EL displayapparatus.

[0028]FIG. 2 is a cross-sectional view of the EL display apparatus ofthe present invention.

[0029]FIGS. 3A and 3B are cross-sectional views of the EL displayapparatus of the present invention.

[0030]FIG. 4 is a cross-sectional view illustrating a problem of aconventional EL display apparatus.

[0031]FIG. 5 is a top view of an active-matrix EL display apparatus ofthe present invention.

[0032]FIG. 6 is a cross-sectional view of the active-matrix EL displayapparatus of the present invention.

[0033]FIGS. 7A and 7B are enlarged cross-sectional views showing theedge of the first electrode of the present invention.

[0034]FIGS. 8A, 8B, and 8C are cross-sectional views showing a formationmethod of the first electrode of the present invention.

[0035]FIG. 9 is a top view and a cross-sectional view of a simple-matrixEL display apparatus of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] A first embodiment of the present invention will be describedhereinafter. The first embodiment is an example applying the presentinvention to an active-matrix organic EL display apparatus. One displaypixel of the EL display apparatus of the first embodiment is shown inFIG. 5 and a cross-sectional view along line A-A in FIG. 5 is shown inFIG. 6.

[0037] A driver circuit for each pixel is identical to the circuit shownin FIG. 1, and the difference with the prior art shown in FIGS. 2, 3A,and 3B is the cross-sectional configuration of an anode 1, or firstelectrode.

[0038] The gate signal line 51, the data signal line 52, the drivingpower supply 53, the electrode 54, and the anode 1 respectivelycorrespond to the gate signal line Gn, the data signal line Dm, thedriving power supply 150, the electrode 172 of the storage capacitor170, and the anode 161 of the organic EL element 160. The gate signallines 51 are arranged in rows and the data signal lines 52 and thedriving supplies 53 are arranged in columns. A capacitor and an emissivelayer are arranged within the area that is partitioned by the signallines and power supply lines. The storage capacitor is formed from thesemiconductor film 13 and the electrode 54. The semiconductor film 13 isconnected to the data signal line 52 via the contact C1, and the gateelectrode 11 is arranged between the drain 13 d and the source 13 s.

[0039] The semiconductor film 43 is connected to the driving powersupply 53 via the contact C2, and the gate electrode 41, which isconnected to the semiconductor film 13, is arranged between the drain 43d and the source 43 s. The semiconductor film 43 is connected to theanode 1 of the organic EL element via the contact C3.

[0040] As shown in FIG. 6, the organic EL display apparatus is formed bylaminating in sequence a TFT and an organic EL element on the substrate10, such as a substrate formed from glass or synthetic resin, aconductive substrate, or a semi-conductive substrate. However, when aconductive substrate or a semi-conductive substrate is used for thesubstrate 10, an insulating film of SiO₂ or SiN is formed on thesubstrate 10, upon which the TFT and organic EL element are formed.

[0041] In the present embodiment, a first TFT 30 and a second TFT 40 areboth so-called top-gate TFTs provided with a gate electrode at the topof the active layer, and a case is given where a semiconductor filmformed from poly-silicon is used for the active layer. Furthermore, thecase is given where the TFT has the gate electrode 11 with a double-gatestructure.

[0042] The first TFT 30, which is a switching TFT, will be describedfirst.

[0043] As shown in FIG. 6, on the insulating substrate 10, which isformed from quartz glass, non-alkaline glass, or the like, are formed insequence the semiconductor film 43 and the gate insulating film 12. Thesemiconductor film 43 is the active layer of the second TFT, and has thesource 43 s, the drain 43 d, and the channel 43 c. On the gateinsulating film 12 is formed the gate electrode 41, which is formed froma refractory metal such as chromium (Cr), molybdenum (Mo), or the like,on which is covered with and formed the interlayer insulating film 15,which is formed by laminating in sequence a SiO₂ film, a SiN film, and aSiO₂ film. Thereon is formed the data signal line 52 and the drivingpower supply 53.

[0044] The TFT has a so-called Lightly Doped Drain (LDD) structure.Namely, ion doping is performed using the gate electrode 41 on a channel13 c as a mask. Furthermore, the gate electrode 41 and an area up to afixed distance from both sides of the gate electrode 41 are covered withresist, and ion doping is performed again to provide a low concentrationarea on both sides of the gate electrode 41 and beyond these areas thesource 43 s and the drain 43 d of a high concentration area.

[0045] Furthermore, the planarization insulating film 17, which isformed from an organic resin or the like, is formed on the entiresurface so as to planarize the surface. A contact hole is then formed ata position corresponding to the source 43 s in the planarizationinsulating film 17, and a transparent first electrode formed from ITOand contacting the source 43 s via the contact C3, namely, the anode 1of the organic EL element, is formed on the planarization insulatingfilm 17.

[0046] The emissive element layer 66 adopts a common structure and isformed by laminating in sequence the anode 1 formed from a transparentelectrode, such as ITO, the first hole transport layer 62 formed fromMTDATA (4,4′,4″-tris (3-methylphenylphenylamino)triphenylamine), thesecond hole transport layer 63 formed from TPD (N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), the emissive layer 64formed from Bebq₂ (bis(10-hydroxybenzo[h]quinolinato) beryllium)including an inductor Quinacridon, the electron transport layer 65formed from Bebq₂, and the cathode 67 formed from a magnesium-indiumalloy or a magnesium-silver alloy or a lithium fluoride-aluminumlamination.

[0047] Furthermore, in the organic EL element, holes injected from theanode and electrons injected from the cathode recombine within theemissive layer, and the organic molecules included in the emissive layerare excited to yield exitons. Light is released from the emissive layerin the process where the exitons undergo radiation deactivation, andthis light is released to the outside from the transparent anode via thetransparent insulating substrate to make the light emission visible.

[0048] Arranging display pixels configured in this manner in a matrix onthe substrate 10 forms an organic EL display apparatus capable ofdisplaying a desired overall image by controlling each pixel.

[0049] The anode 1 of the present embodiment has edges forming taperedinclines as shown in FIG. 6. Due to these inclines, the emissive elementlayer 66 is smoothly formed from the anode 1 on the planarizationinsulating layer 17, thereby preventing the coverage from deterioratingand the anode 1 and the cathode 67 from shorting. Furthermore, since theinclines become broader on the substrate side, there are no sharp edgeson the top edge of the anode 1 facing the cathode 67, making fieldconcentration less likely to occur. Therefore, the emissive layer 64emits light uniformly on the entire surface and partial deteriorationdoes not occur rapidly.

[0050] It is preferable for the angle θ formed from the incline of theanode 1 shown in FIGS. 7A and 7B with the plane of the planarizationinsulating film 17 to be small so as to prevent rupture or fieldconcentration. However, if the angle θ is too small, the edge of theanode 1 becomes extremely thin so that a problem arises wherereproducibility of the shape decreases. Therefore, the angle formed bythe plane of the bottom layer or the substrate 10 with the inclined sidefaces of the anode 1 is set from 10° to 45°, and preferably around 30°.Furthermore, it is preferable for the top edge of the anode 1 to have asmooth curve as shown in FIG. 7B.

[0051] A method for forming the anode 1 into an incline will bedescribed next. As described above, although the etching of the ITO filmemployed the conventional wet etch method, the angle θ of the inclinebecomes substantially 90°. In the present embodiment, a positivephotoresist is formed on the ITO film, which has been formed on theentire surface, and dry etching is performed using a chlorine-based gas,such as Cl₂ or HCl, to form an incline on the ITO edge. FIGS. 8A to 8Care cross-sectional views showing the formation method of the anode 1.First, as shown in FIG. 8A, an ITO film 21 is formed on the entiresurface of the planarization insulating film 17. Next, a positivephotoresist 22 is formed at a predetermined area. When this is exposedto a chlorine-based gas, such as chlorine gas or hydrogen chloride gas,the ITO film 21 and the photoresist 22 are etched isotropically. Withdry etching using chlorine-based gas, the selectivity is low between theITO film 21 and the photoresist 22 so that the photoresist 22 is etchedsimultaneously with the ITO film 21. However, since etching is fasterfor the ITO film 21, the etching proceeds as shown in FIG. 8B eventhough the selectivity is low. The etching continues and completes asshown in FIG. 8C. In the present embodiment, the angle θ of the inclinebecomes approximately 30°. In this manner, the isotropic etching isperformed using an etching gas having low selectivity between the ITOfilm and the resist so that the anode 1 is formed with sloping edges.

[0052] The film thickness of the anode 1 is described next. The filmthickness of the anode 1 is thinly formed compared to the total filmthickness of the emissive element layer 66. When the film thickness ofthe anode 1 is thin, a step developing between it and the planarizationinsulating film 17 is reduced so that a rupture of the emissive elementlayer 66 can be prevented. Since the color of the display changesdepending on the thickness of the anode 1, an arbitrary thickness cannotnecessarily be set. The film thickness of the anode 1 is set to ½ thetotal thickness of the emissive element layer 66 or less if possible,and preferably to ⅓ or less. However, if the anode 1 is formed too thin,the reproducibility of the shape decreases due to chipping of part ofthe anode 1 and so forth. In the present embodiment, the anode 61 has athickness of approximately 85 nm, the emissive element layer 66 has atotal thickness of approximately 200 nm, and the cathode 67 has athickness of approximately 200 nm.

[0053] The present invention is also applicable to a simple-matrix ELdisplay apparatus. FIG. 9 shows a top view and a cross-sectional viewalong line A-A of the simple-matrix EL display apparatus representing asecond embodiment of the present invention.

[0054] Arranged on a transparent substrate 70 are an anode 71, which isa first electrode extending longitudinally, and a cathode 72, which is asecond electrode extending transversely and crossing the first electrode71. In the emissive element layer 66, the emissive layer 64 is formed ateach intersection of the anode 71 and the cathode 72.

[0055] Although the TFT was illustrated in the aforementionedembodiments as having the top-gate structure in which the gate electrodeis located on the active layer, it may have a bottom-gate structureinstead. Furthermore, although a semiconductor film was used for theactive layer in the aforementioned embodiments, a micro-crystallinesilicon film or amorphous silicon may be used instead.

[0056] In this embodiment also, the edge of the anode 71 inclines andbecomes broader toward the substrate so that the emissive element layer66 smoothly covers the anode 71, thereby preventing shorts between theanode 71 and the cathode 72.

[0057] Furthermore, although an organic EL display apparatus wasdescribed in the aforementioned embodiments, the present invention isnot limited thereto, and may be also applicable to an inorganic ELdisplay apparatus having a emissive layer formed from inorganicmaterials, while yielding a similar effect.

[0058] Furthermore, although the first electrode was described in thepresent specification as an anode, the first electrode is arrangedbetween the substrate and the EL element (EL layer) and is the electrodecovered by the EL layer so that in some cases it may be a cathode.

[0059] While there has been described what are at present considered tobe preferred embodiments of the invention, it will be understood thatvarious modifications may be made thereto, and it is intended that theappended claims cover all such modifications as fall within the truespirit and scope of the invention.

What is claimed is:
 1. An electroluminescence display apparatuscomprising: a first electrode formed above a substrate; an emissiveelement layer formed on said first electrode; and a second electrodeformed on said emissive element layer; side faces of said firstelectrode are inclined and become broader toward the substrate side. 2.An electroluminescence display apparatus according to claim 1 whereininclined side faces of said first electrode has an angle from 10 degreesto 45 degrees with respect to the plane of the lower layer and/or thesubstrate.
 3. An electroluminescence display apparatus according toclaim 1 wherein inclined side faces of said first electrode has an anglefrom 25 degrees to 35 degrees with respect to the plane of the lowerlayer and/or the substrate.
 4. An electroluminescence display apparatusaccording to claim 3 wherein the thickness of said first electrode isless than ½ the film thickness of said emissive element layer.
 5. Anelectroluminescence display apparatus according to claim 3 wherein thethickness of said first electrode is less than ⅓ the film thickness ofsaid emissive element layer.
 6. An electroluminescence display apparatusaccording to claim 1 wherein the thickness of said first electrode isless than ½ the film thickness of said emissive element layer.
 7. Anelectroluminescence display apparatus according to claim 1 wherein thethickness of said first electrode is less than ⅓ the film thickness ofsaid emissive element layer.
 8. An electroluminescence display apparatusaccording to claim 1, wherein said first electrode is unique to a pixel,and the apparatus is an active-matrix type having a thin-film transistorfor driving said emissive element.
 9. An electroluminescence displayapparatus according to claim 8 further comprising a planarizationinsulating film formed so as to cover said thin-film transistor, withsaid first electrode formed on said planarization insulating film. 10.An electroluminescence display apparatus according to claim 1 is apassive-matrix type wherein said first electrode extends in a firstdirection and said second electrode extends in a second direction so asto intersect said first electrode.
 11. An electroluminescence displayapparatus according to claim 1 wherein said emissive element layer islaminated with a hole transport layer, an emissive layer, and anelectron transport layer.
 12. An electroluminescence display apparatuscomprising: the first electrode formed above a substrate; the emissiveelement layer formed on said first electrode; and the second electrodeformed on said emissive element; the thickness of said first electrodeis less than ½ the thickness of said emissive element layer.
 13. Anelectroluminescence display apparatus comprising: the first electrodeformed above a substrate; the emissive element layer formed on saidfirst electrode; and the second electrode formed on said emissiveelement; the thickness of said first electrode is less than ⅓ thethickness of said emissive element layer.
 14. An electroluminescencedisplay apparatus according to claim 12 is an active-matrix typecomprising said first electrode formed independently at each pixel, andthin-film transistor for driving said emissive element.
 15. Anelectroluminescence display apparatus according to claim 14 furthercomprising the planarization insulating film formed so as to cover saidthin-film transistor, with said first electrode formed on saidplanarization insulating film.
 16. An electroluminescence displayapparatus according to claim 14 wherein said emissive element layer islaminated with a hole transport layer, an emissive layer, and anelectron transport layer.
 17. An electroluminescence display apparatusaccording to claim 12 is a passive-matrix type wherein said firstelectrode extends in a first direction and said second electrode extendsin a second direction so as to intersect said first electrode.
 18. Anelectroluminescence display apparatus according to claim 17 wherein saidemissive element layer is laminated with a hole transport layer, anemissive layer, and an electron transport layer.